1. Cross-Reference to Related Applications
This application relates to co-pending U.S. patent application Ser. No. 08/915,090, attorney docket number M-4932 US, filed on even date herewith, entitled "Combination Electromagnetic Shield and Heat Spreader" and naming Charles D. Hood, III, Damon W. Broder, and Eric B. Holoway as inventors, the application being incorporated herein by reference in its entirety.
2. Field of the Invention
The present invention relates to electromagnetic shielding for computer systems and more specifically to providing a low impedance electrical connection for an electromagnetic shield.
3. Description of the Related Art
An electromagnetic shield is typically a metallic partition placed between two regions of space. The electromagnetic shield controls the propagation of electric and magnetic fields from one of the regions to the other. An electromagnetic shield may be used to contain electromagnetic fields if the shield surrounds the source of the electromagnetic fields.
A solid electromagnetic shield that completely surrounds a product can be at any potential and still provide effective electromagnetic shielding. That is, the shield prevents outside influences from affecting circuits inside the electromagnetic shield and vice versa. Thus, the electromagnetic shield need not to be grounded or have its potential defined in any way. However, an ungrounded or undefined electromagnetic shield should completely enclose the object being protected and that object being protected should have no connection to the outside world.
In practice, however, the electromagnetic shield is not a complete enclosure, and the object inside does have connections to the outside world, either directly, through signal and/or power leads, or indirectly, through stray capacitance due to holes in the electromagnetic shield. In such cases, the electromagnetic shield should be grounded or have its voltage potential defined with respect to the noise source to prevent the noise source's potential from coupling to the enclosed object. An ungrounded or undefined electromagnetic shield's potential varies with conditions and location, and therefore the noise coupled to the object inside also varies.
Grounding also has other benefits. Grounding provides a path for radio frequency (RF) currents to flow on the structure. Grounding also prevents the buildup of AC potentials on the equipment enclosure. Grounding provides a fault-current return path to protect personnel from shock hazards. Grounding also prevents the buildup of static charge.
The electromagnetic shield should have a low-impedance coupling with a voltage reference such as a ground plane of a printed circuit board in at least two places in order to properly define the voltage potential or ground the electromagnetic shield in a computer system. However, today's computer systems include high frequency electromagnetic sources such as processors which may require the electromagnetic shield to be electrically coupled to a voltage reference such as a ground plane at several locations. The higher frequencies of the electromagnetic sources require closer spacings between the grounding connections of the electromagnetic shield to a voltage reference in order to provide effective electromagnetic shielding. Coupling a generally planar electromagnetic shield at several closely spaced locations around its perimeter allows an electromagnetic shield to form the top portion of an effective electromagnetic shield enclosure with a ground plane forming the bottom portion.
Screws, star washers, thread-cutting screws, soldering, grounding clips, or other types of grounding connectors can be used to provide a low impedance coupling. However, these methods can be expensive and increase the complexity of the manufacturing a circuit board, especially as more grounding connections are used for a given computer system.
What is needed is a simple and cost efficient way to provide a low impedance electrical coupling of an electromagnetic shield to a voltage reference such as a ground plane.